Asphalt is a general term that refers to the various bituminous substances that are used extensively for paving and road making. Asphalt binders function as adhesion promoters for asphalt mixtures or aggregates and are typically comprised of naturally occurring hydrocarbons or petroleum distillate residue with or without polymer or chemical modifiers. In the paving industry, the term “aggregate” is used for a mass of crushed or uncrushed stone, gravel, sand, etc., predominantly composed of individual particles, but in some cases including clays and silts. The performance characteristics of asphalt binders are of particular importance in roadway construction. Understanding the limitations of the materials used for roadway construction permits the design and construction of roadways that are more stable, durable, and that offer greater safety to the user of the roadway.
Low temperature thermal shrinkage cracking is one of four major failure modes in asphalt pavement, together with rutting, fatigue cracking, and moisture damage. Thermal shrinkage cracking in asphalt pavement occurs when the thermal tensile stress within the asphalt pavement that results from temperature drop exceeds the strength at that temperature. Thermal cracks typically appear as transverse cracks (pavement cracks perpendicular to the direction of traffic) at regular intervals in the field pavements.
Historically, thermal cracks occurring at low temperatures have been controlled by limiting the asphalt binder stiffness. Assuming similar asphalt binder tensile strengths and coefficients of thermal expansion/contraction, the binders with a higher stiffness will crack at a higher temperature than softer binders. Because an accurate and easy to use measuring instrument was not available, the cracking temperature or the limiting low temperature stiffness of asphalt binder had been extrapolated from consistencies measured at higher temperatures, such as penetrations at 5 and 25° C., viscosity at 25° C., or ring-and-ball softening point (50-60° C.). Hill, J. F., Inst. Petroleum, vol. 74-014 (1974) and Van der Poel, C., Journal of Applied Chemistry, vol. 4, 221-236 (1954).
In the United States, the Association of American State Highway and Transportation Officials (AASHTO) has published and implemented a series of performance graded (“PG”) binder specifications. These specifications were the result of the Strategic Highway Research Program (SHRP) which was conducted from 1987-1994. The SHRP/AASHTO system for specifying asphalt binders is unique in that it is a performance-based series of specifications. Various binders are categorized on the basis of the climate and attendant pavement temperatures at which the binder is expected to operate.
Under this system, physical property requirements remain the same, but the temperature at which the binder must attain the properties, changes. For example, a binder graded as PG 64-22 possesses adequate physical properties up to 64° C., which would be the high pavement temperature corresponding to the climate in which the binder is expected to operate. Similarly, the PG 64-22 binder possesses adequate physical properties down to at least minus 22° C. Thus, as illustrated by this example, the thermal characteristics of an asphalt binder are central to this grading system.
As will be appreciated by those skilled in the art, low-end temperatures of PG grading are typically determined by utilizing one or more of several known systems including the Bending Beam Rheometer (BBR) and/or the Direct Tension Tester (DTT). While effective at generating useful data, these systems are complex, require the performance of numerous calculations, require the testing of many specimens, do not directly measure the temperature at which the specimen fails, and are often very time consuming and expensive to perform. Thus, there is a need for a low-cost device and method that quickly and accurately characterizes the critical thermal characteristics of asphalt binder and aggregate specimens.